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The atmospheric deposition of arsenic and association with acid precipitation
Atmospheric EnvironmentVol. 22, No. 5, pp. 937-943, 1988.
0004-6981/88 $3.00+0.00 © 1988 Pergamon Press pie
Printed in Great Britain.
THE ATMOSPHERIC DEPOSITION OF ARSENIC AND ASSOCIATION WITH ACID PRECIPITATION JOSEPH R. SCUDLARKand THOMAS M. CHURCH University of Delaware, College of Marine Studies, Lewes, DE 19958-1298, U.S.A. (First received 13 July 1987 and in final Jbrm 19 October 1987)
Abstract--Measurements of As concentration and speciation in acid precipitation events were conducted at the mid-Atlantic coast oftbe U.S for the period June 1985-October 1986. Only inorganic As was detected, in concentrations ranging from <0.01 to 14.95 nmolel-1, with an annual volume-weighted mean of 1.30 nmole 1-1. The total As concentration exhibited a positive correlation with acidity, excess (non-sea-salt) sulfate, and nitrate, suggesting common atmospheric sources and fates from fossil fuel combustion. Contributions of As from marine sources appear to be minor. Reduced As (111) was found to comprise between 0 and 40 % of the total in selected events; the As(lI1)/(V)ratio may be indicative of the redox intensity (pE) and/or source of the associated storm system. Based on concurrent bulk (wet plus dry) collections, the total As deposition rate varied from 42.7 to 296 nmole m 2 mo- ~,with dry deposition estimated to comprise between 29 and 55 % of the total. Key word index: Acid rain, arsenic, tracers, chemistry of precipitation, pollution sources.
be sampled without significant ground-level contamination, which can often be a problem with groundWhile the utility of As as an elemental tracer of coal based aerosol and dry deposition measurements. combustion has previously been recognized (Walsh While atmospheric As can be derived from both et al., 1979b; Gordon, 1980; Rahn and Lowenthal, natural and anthropogenic sources (Pierson et al., 1984), studies to date have exclusively focused on the 1973; Brimblecombe, 1979; Lantzy and Mackenzie, ambient aerosol composition. However, when such an 1979; Walsh et al., 1979b), in the northeast U.S the elemental tracer approach is utilized to identify the atmospheric burden is dominated by industrial emissource(s) of acid rain, there are a number of distinct sions. The major source of such emissions is from the advantages to examining such tracers directly in pre- combustion of coal, including electric power genercipitation. First, in precipitation such tracers are ation and non-ferrous smelting (Walsh et al., 1979b). subjected to similar emission, transport and deposition As in coal fly ash exists primarily as the oxide, processes as the acidic components. Evidence suggests As(llI)20 3 (Crecelius, 1975; Turner, 1981), and like that fine atmospheric particulates, typical of trace sulfate, is associated with fine sub-/~m aerosols (Linton metal combustion condensates and acid aerosols, are et al., 1976; Walsh et al., 1979a). There is also evidence most effectively removed from the atmosphere by wet for the existence of an atmospheric vapor phase (Walsh deposition (Arimoto et al., 1983). Second, layering in et al., 1979a), which is perhaps related to biogenic the lower troposphere (leading to chemical stratifi- emissions of methylated forms (Johnson and Pilson, cation in the composition of the atmosphere) is particu- 1975). larly important in addressing the sources of acid Concentrations of As in precipitation reported precipitation, since long-range transport and in-cloud for various world-wide locations (as summarized nucleation processes occur in conjunction with upper by Galloway et al., 1982a) range from 0.07 to level air flow. While precipitation acts to vertically 77 nmole 1-1. Recent studies of As in rainwater from integrate the lower troposphere with respect to alti- the west coast of the U.S. and Hawaii (Andreae, 1980) tude, most aerosol measurements only reflect con- reveal relatively low concentrations (averaging ditions at or near ground level. Third, in contrast to 0.25 nmolel-~), consistent with open ocean aerosol aerosol measurements during periods of typical air measurements (Duce et al., 1975; Walsh et al., 1979a) flow, precipitation events at a given location tend to and onshore air flow. In contrast, precipitation and exhibit very specific storm trajectories, and in a aerosols collected downwind from a large copper meteorological sense, are very ephemeral features smelter exhibit relatively high (13-227 nmole 1-1) As (usually precipitation occurs less than 10 '.~oof the time). concentrations (Crecelius, 1975; Andreae, 1980). Thus, it is unlikely that aerosol measurements made Limited east coast measurements (Waslenchuk, 1978; during non-precipitating periods are representative of Galloway et al., 1982a) exhibit large variability the sources of acid rain precursors. Lastly, we have (0.4-27 nmole 1-1). Thus, due to variable and limited shown (Tramontano et al., 1987) that precipitation can representative data for As deposition over continental INTRODUCTION
937
938
JOSEPH R. SCUDLARK and THOMAS M. CHURCH
regions, attempts to obtain a global atmospheric mass balance for As (Lantzy and Mackenzie, 1979; Walsh et al., 1979b; Andreae, 1980) suffer from a large degree of uncertainty.
out of 90 total were analyzed, representing 90 ~'. of the total incident precipitation during this period. Due to volume limitations, only 15 of these event,~ werc analyzed for reduced As(Ill). Bulk deposition collections and measurements were conducted for the period October 1985-October 1986 only.
SAMPLING AND ANALYSES RESULTS AND DISCUSSION
Precipitation samples for As analysis were obtained on an event basis at the Lewes, DE, MAP3S/WATOX site using a wet-only sampler and rigorous trace metal sampling protocol (Tramontano et al., 1987). The sampling site is located on the mid-Atlantic coast, in a remote area of Cape Henlopen State Park (38 ° 46'N, 75°06'W). The Atlantic Ocean is located about 0.8 km to the east, while Delaware Bay is about 2 km to the northwest. The soil in the vicinity of the site is composed of well-sorted, coarse sand and the vegetation is predominantly scrub pine. Precipitation at this site is generally representative of the northeastern U.S. acid rain corridor, with an annual average pH of about 4.25 and average volume-weighted sulfate and nitrate concentrations of 24.5 and 18.9 pmole 1-~, respectively (Church et al., 1982; Dana and Easter, 1987). Due to its maritime location, many events inherit a variable marine component as well, with an average of 14 ',"oof the total sulfate being derived from sea-salt incorporation. Collections utilize a commercially available automatic collector (Aerochem Metrics, Inc., Bushnell, FL) modified with polycarbonate lid and Teflon-coated support arms, lining of the collection bucket with a heavy gauge (4rail) polyethylene bag liner, postacidification (0.4 ~."ov/v) of the sample in the collector with high-purity HCi to effect quantitative desorption, and the use of acid-cleaned liners and polyethylene storage bottles (Tramontano et al., 1987). Concurrent bulk deposition (wet plus dry) measurements were made using an acid-washed polyethylene funnel-bottle sampler which was continually exposed to the atmosphere. The funnel possesses extended vertical sides (32 cm) which are intended to limit aerodynamic effects on sampling efficiency. The funnel orifice was located about 2 m above ground level, approximately the same height as the wet-only collector. Arsenic speciation determinations were made using the d.c. plasma emission spectrometry procedure of Braman et al. (1977), except that high-purity HCI was substituted for pH adjustment in the total As analysis. Samples for major ions and reduced S(IV) were collected in parallel with the trace element samples and assayed according to standard MAP3S Network procedures (Dana and Easter, 1987). Due to oxidation of arsenite resulting from the prescribed acidification of trace metal samples, separate aliquots for reduced As(Ill) determination were obtained from the major ion sampler, and stored frozen until analysis. Field and laboratory blanks were established to verify the accuracy of our measurements. For the period June 1985-October 1986, 74 events with sufficient volume
In all samples analyzed, only inorganic As was detected. About 25 '!.,;of the samples were analyzed for organo-As species (methylarsonate and dimethylarsinate), selected to represent extremes in pH, season, and storm trajectory; in none of the samples were these species present above our detection limits for these compounds (0.10 nmolel ~ ~/. Despite the presence of relatively high concentrations of these biomethylated arsenic species in nearby coastal waters (Sanders, 1985) and their chemical stability at low pH values typical of local precipitation (Andreae, 1980), these compounds do not appear to make a significant contribution to the atmospheric deposition of As at our site. Concentrations of total inorganic As measured in precipitation events (Table 1) range from <0.07 to 14.95 nmolel - t , with an annual volume-weighted mean of 1.30 nmolel-~. Perhaps the most revealing observation is the correlation between total As with acidity (proton concentration) and with the acid anions, nitrate and excess (non-marine) sulfate (Figs la-c). These correlations suggest that As and the acid components share common transport and deposition pathways; similar observations have been made for Se at this same site (Cutter and Church, 1986). The consistent positive As intercepts in Figs la-c (0.61 __+0.16 nmole 1- ~) may reflect a regional background input of As from non-acidic sources, such as from crustal weathering or from biogenic emissions of volatile methyl arsenic compounds (Johnson and Braman, 1975). The concentration of each major ion (Junge, 1963) and trace element (Lindberg, 1982) in precipitation is influenced by many factors, including its atmospheric aerosol concentration, its in- and below-cloud scavenging efficiency, and the precipitation type, amount, duration, intensity, and antecedent dry period. With the acid components and As, a weak inverse relationship exists between the rainfall amount and concentration (an exponential decrease in concentration with increased precipitation), suggesting that their concentrations may be related to the quantity of precipitation. However, with the exception of Se (Cutter and Church, 1986) and NH~, no other major ion or trace element we have examined exhibits strong correlations with the acid species. Thus, in consideration of what is known about the atmospheric sources of As (Brimblecombe, 1979; Lantzy and Mackenzie, 1979; Walsh et al., 1979b), we believe these correlations reflect common emission sources, and are not simply the result of a common washout/dilution effect related to rainfall amount.
Deposition of arsenic and association with acid precipitation
939
Table I. Arsenic deposition at the mid-Atlantic coast of North America Volume-weighted average concentration in precipitation: Average wet depositional flux: Average (measured) dry depositionai flux: Average (calculated) dry depositional flux:
1.30 nmolel1.49 ,amolem- 2 a- l 0.61 ,amolem-2 a 1.15 ,amolem-2 a -j
Average (measured) total (wet + dry) deposition rate: Average total As in air: Washout ratio (,agkg- 1) precip./(,ag kg- ~) air: Crustal enrichment factors (AI): Precipitation Aerosols
2.10 ,amolem-2a 1 1.05 ng m - 3
110 25 31
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Fig. 1. Correlation of arsenic (nM) with (a)protons Lueql-~); (bl excess (non-sea-salt, sodium normalized) sulfate (,aM); (c) nitrate (/aM); and (d) sodium (,aM) in precipitation at the mid-Atlantic coast. (b) [A,], o.ozgz [sq,']. o.Tz r = 0.63,
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While some limited measurements of As in atmospheric aerosols indicate predominance in the oxidized pentavalent state (Waslenchuk, 1978), other studies (Crecelius, 1975; Andreae, 1980; Turner, 1981) have reported that As emissions in coal fly ash and from smelting activities are almost entirely in the reduced trivalent state, presumably As(III)20 3. In the precipitation samples we have analyzed for As(Ill), we have detected no more than 40'>o of.the total As present as arsenite (Fig. 2). In addition, a significant fraction of these samples exhibited As(Ill) concentrations below our detection limits ( < 0.007 nmole 1-1), though these results may be somewhat biased since only larger volume events were analyzed. No correlation was observed between the As(III)/(V) ratio and any other trace element analyzed [including the Se(IV)/(VI) ratio]; none of the samples analyzed for As(Ill) revealed detectable levels (<0.1 #M) of S(IV). In order to maintain the proportionality between the precipitation acidity and total As concentration (Fig. 1), the wide distribution in As oxidation states shown in Fig. 2 cannot be explained simply on the basis of temporal variability in the emission source strengths of the As redox species. Though no published data exist on the rate of arsenite oxidation in rainwater, rates reported for other aqueous systems (Johnson and
940
JOSEPH R. SCtlDLARK and THOMAS M. ('HURCH
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H+ (ju equiv.I-I) Fig. 2. As(III)/(V) ratio vs protons (.ueql ~} in precipitation at the mid-Atlantic coast.
(1) In accord with the observed As-H * relationship (Fig. la), storms influenced by marine air masses (which are characterized by comparatively high pH values, typically 4.50-6.00), exhibit the lowest arsenic levels. (2) Direct measurements of storms exclusively of oceanic origin sampled at Lewes (Hurricanes Danny, Gloria, and Charley, tropical storm Henri) display relatively low As concentrations ( <0.40 nmolel ~). These results are corroborated by analyses of snow pit samples from Antarctica and Greenland at our laboratory and elsewhere (Weiss et al., 1975), which also exhibit low background As concentrations ( < 0.27 nmolel ~). However. as previously discussed, the low concentrations in oceanic storms may partially reflect increased dilution by typically larger volume events. (3) A lack of correlation between As and Na (Fig. ld) indicates that processes generating sea-sah aerosols do not contribute to As in precipitation. Based on seawater As concentrations reported for the western North Atlantic (Johnson and Pilson, 1972) and the average Na concentration in local precipitation (Church et at., 1982), the direct input of inorganic As from seasah incorporation (assuming no chemical fractionation) would be insignificant ( < 1 ",,).
Pilson, 1975; Cherry et al., 1979; Tallman and Shaikh, 1980; Oscarson et al., 1981), adjusted where possible to reflect a precipitation matrix (with respect to ionic strength, temperature, pH, and initial arsenite concentration), are not nearly sufficient to account for the observed As species distribution. Thus, it appears that the As speciation in precipitation is mediated by the redox poise established by atmospheric oxidative catalysts (e.g. 03, H202, u.v. light, other trace metals such as Mn and Fe) and reductants (e.g. SO2, HSO3) Based on the As-H ~ relationship described here (Oscarson et al., 1975; Andreae, 1980; Taliman and .(Fig. la), and the precipitation acidities reported for Shaikh, 1980). remote global locations [5.0 < pH < 5.6; Galloway et Based on the measured As(III), As(V) and H ~ al. (1982b)], we would predict open-ocean As concenconcentrations, it is possible to calculate an apparent trations in precipitation in the range 0.5~).6 nmole I - t. redox intensity, pE = - log{e- ]. The appropriate half- This estimate is in good agreement with measured cell reaction dictated by the p H - p E domain of precipi- (Andreae, 1980) and predicted (Walsh et al., 1979b) tation is precipitation concentrations for remote sites. A similar extrapolation based on average annual precipitation H3As(III)O3 + n 2 0 ~ H2As(V)O4- + 3H + + 2eacidities reported for the northeastern U.S. [4.0 < pH E = 0.649 V < 4.2; Dana and Easter 11987)] yields As concen[As(Ili)] trations < 1.8 nmolel- ~. On this basis, our predicted so that: pE = I 1.0 - 3,/2 pH - 1:/2 log [As(V)] concentration is about a factor of two less than the median average reported for rural U.S. locations [as (Cherry et al., 1979). reviewed by Galloway et aL (1982a)]. Though this The mean pE calculated accordingly (5.29 + 0.68, discrepancy might be explained in part by geographical n = 14) is about half that similarly estimated for the variability, some of the older data on trace metals in Se(IV)/(VI) couple [11.50 + 1.01, Cutter and Church, wet deposition are suspec! because of the lack of (1986)-], and approximates that reported for the proper precautions in avoiding sample contamination N O 2 / N O ~- couple in rainwater (Morgan, 1982). Thus, (Barrie et al., 1987). While there exists no widely accepted method for As speciation appears to equilibrate faster and is more "labile" in terms of its redox response during atmos- accurately measuring dry deposition by direct pheric transport, while Se speciation is more resistant methods, our bulk measurements provide an indirect estimate of the total (wet plus dry) As deposition. to oxidation (conservative). Overall, our results lead us to conclude that As in These data indicate that the total arsenic deposition precipitation has a dominant anthropogenic source. rate along the mid-Atlantic coast (Fig. 3) ranges from Arsenic of marine origin (such as that derived from 42.7 to 352 nmole m 2 mo ~, with an annual average natural volatile biogenic sources and/or fractionation of 2.1 #mole m 2 a 1. Dry deposition of As (estimated from seawater) appears to make a minor contribution as the difference between concurrent bulk and wetto the atmospheric depositional flux, which has been only collections) comprises between 2 and 77 "~, of the suggested by other researchers as well (Waish et al., monthly averaged total As deposition (Fig. 3). 1979b). There are several lines of experimental evidence However, because of the abnormally low amount of rainfall during the summer of 1986 (about 25 ".,, below which support .our contention:
Deposition of arsenic and association with acid precipitation
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J A S 0 ND J F MAMJ J 1985 I[Z2~ m. m ~11986 MONTH
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941
signatures for the midwestern U.S. developed by Dutkiewicz et al. (1987), but not Rahn and Lowenthars (1984) midwestern source region As:Se ratio. We are currently conducting a more thorough examination of the use of such key elemental ratios in aerosols and precipitation in conjunction with air parcel trajectory analysis to define regional chemical signatures relative to our Lewes, DE, sampling site. The degree of atmospheric enrichment of As in Lewes, DE, precipitation and aerosols can be compared with that of other metals based on calculated enrichment factors (EF), i.e.
E F (A1) = ( ~ l )
Fig. 3. Monthly trends in wet and dry arsenic deposition lnmole m- 2 mo- 1 at the mid-Atlantic coast.
average), our dry deposition rates for this period may be slightly over-estimated. Overall, on a seasonal basis the wet and dry As deposition rates parallel the patterns regularly observed with the acidic components (summer maximum, winter minimum). As summarized in Table 1, the average measured dry deposition rate (50.7 nmole m -2 t o o - i ) is only about half the rate calculated on the basis of the average measured As aerosol concentration at Lewes (1.05 ng m-3, Wolff et al., unpublished data; Zoller et al., unpublished data) and reported particle deposition velocity for As (0.24 cms-1; Pierson et al., 1973). This would suggest that (a) measurements from our bulk collector are not significantly compromised by contamination from resuspended soil or other fugitive debris; (b) despite the occurrence of As aerosols as sub-#m particles [ M M D = 0.63 _+ 0.24/~m; Walsh et al. (1979a)], coupled with acidified (pH -<_1.6) storage of samples prior to analysis, some particulate As may be refractory and not detected by our methods; and/or (c) due to aerodynamic effects, our bulk sampler did not efficiently collect dry fallout. Overall, it seems reasonable to assume that the dry deposition of As represents between 29 % (measured) and 55 % (calculated) of the total As deposition at our site, which is similar to values reported for most other atmospheric trace metals (Galloway et al., 1982). Based on principal component analyses of aerosol data, Wolff et al. (1986a)conclude that coal combustion emissions from the midwestern U.S. are the primary source of sulfate at our Lewes site. Since atmospheric Se is also recognized as a tracer of coal combustion emissions, it is noteworthy that the As:Se ratios in Lewes, DE, precipitation [0.59, this work; Cutter and Church, (1986)] and aerosols [0.55; Wolffet al. (1986a), Zoller et al. (unpublished data)] are nearly identical. This agreement indicates that these elements have similar washout ratios (i.e. precipitation scavenging efficiencies), so that precipitation appears to serve as an effective integrator of such trace element emissions. These ratios are consistent with the regional elemental
precip / ( ~ )
shal;
where the concentration of element "X" is compared in precipitation (or aerosols) to its relative crustal abundance (Turekian and Wedepohl, 1961) using A1 as a reference element. When such a comparison is made for trace elements based on aerosol and precipitation data from Lewes (Church et al., 1984; Wolff et al., 1986a), Bermuda (Jickells et al., 1984; Wolff et al., 1986b) and Enewetak Atoll in the South Pacific (Duce et al., 1983), the relative order and magnitude of EF values (Fig. 4) are nearly identical to those reported in a similar comparison between Bermuda and Antarctic aerosol data (Duce et al., 1975). Such spatial and temporal consistency for all elements suggest common mobilization and transport processes leading to global enrichment of the troposphere. Furthermore, the consistency between the aerosol and precipitation enrichment factors for most metals would suggest that precipitation is an effective integrator of the atmospheric particulate composition. Somewhat surprisingly, the EF values for As are consistently lower than most other metals, more closely resembling crustal-dominated elements (EF near unity) than its sister metalloid Se. While the EF values for Bermuda are in agreement with earlier results (Walsh et al., 1979a), those for Lewes are on the 5 ; ~s 4
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.=,i 0
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Fig. 4. Trace element enrichment factors (AI normalized) in precipitation and aerosols at Lewes, DE, Bermuda and Enewetak Atoll.
9~-2
Jos[ PH R. S('[IDLARKand THOMAS M. CH[ RCH
lower end of the range reported for other Northern Hemisphere urban, continental and coastal marine sites (Rahn, 1976: Walsh et al., 1979a). The EF values for As also do not conform 1o the general trend that elements which form oxides with the lowest boiling points (i.e. are the most readily volatalized) exhibit the largest degree of enrichment.
CONCLUSIONS 1. The average concentration o f As in precipitation at our mid-Atlantic site is 1.30 nmolel 2. O f the total (wet + dry) As deposition rate (2.1 l~mole m - 2 a - 1) from 29 ~,, (meas.) to 55 '."~i(calc.j is dry fallout. 3. Atmospheric As at this site has a d o m i n a n t anthropogenic source, and shares c o m m o n transport and deposition pathways with acidic c o m p o n e n t s in precipitation. 4. Marine-derived As appears to be a negligible ( < 10';~o) c o m p o n e n t in precipitation at our site. 5. Redox speciation of As in precipitation is highly variable and may be indicative o f the oxidation intensity of the attendant air mass. Acknowledgements This work was supported by the U.S. Geological Survey (Delaware State Water Resources ProgramJ, the U.S. Department of Energy (MAP3S Network) and U.S. NOAA Air Resources Laboratory (WATOX Program). We thank P. Salevan for sample collection and computer assistance, Dr David L. Johnson for valuable comments on this manuscript and the loan of instrumentation, and W. B. Lyons and the Glacier Research Group for providing polar snow samples. This is a contribution to the WATOX Program. REFERENCES
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Deposition of arsenic and association with acid precipitation method for the collection, handling, and analysis of trace metals in precipitation. Environ. Sci. Technol. 21,749-753. Turekian K. K. and Wedepohl K. H. (1961) Distribution of the elements in some major units of the Earth's crust. Geol. Soc. Am. Bull. 72, 175-192. Turner R. R. (1981) Oxidation state of arsenic in coal ash leachate. Environ. Sci. Technol. 15, 1062-1066. Walsh P. R., Duce R. A. and Fasching J. L. (1979a) Tropospheric arsenic over marine and continental regions. J. geophys. Res. 84, 1710-1718. Walsh P. R., Duce R. A. and Fasching J. L. (1979b) Considerations of the enrichment, sources, and flux of arsenic in the troposphere. J. geophys. Res. 84, 1719--1726. Waslenchuk D. G. (1978) The budget and geochemistry of
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arsenic in a continental shelf environment. Mar. Chem. 7, 39-52. Weiss H., Bertine K., Koide M. and Goldberg E. (1975) The chemical composition of a Greenland glacier. Geochim. cosmochim. Acta 39, 1-10. Wolff, G. T., Kelly, N. A., Ferman, M. A. Ruthkosky, M. S., Stroup, D. P. and Korsog, P. E. (1986a) Measurements of sulfur oxides, nitrogen oxides, haze and fine particulates at a rural site on the Atlantic coast. J. Air. Pollut. Control Ass. 36, 585-591. Wolff G. T., Ruthkosky M. S., Stroup D. P., Korsog P. E., Ferman M. A. and Wendel G. J. (1986b) Sources of SO~, NOx and aerosol species on Bermuda. Atmospheric Environment 20, 1229-1239.
0004-6981/88 $3.00+0.00 © 1988 Pergamon Press pie
Printed in Great Britain.
THE ATMOSPHERIC DEPOSITION OF ARSENIC AND ASSOCIATION WITH ACID PRECIPITATION JOSEPH R. SCUDLARKand THOMAS M. CHURCH University of Delaware, College of Marine Studies, Lewes, DE 19958-1298, U.S.A. (First received 13 July 1987 and in final Jbrm 19 October 1987)
Abstract--Measurements of As concentration and speciation in acid precipitation events were conducted at the mid-Atlantic coast oftbe U.S for the period June 1985-October 1986. Only inorganic As was detected, in concentrations ranging from <0.01 to 14.95 nmolel-1, with an annual volume-weighted mean of 1.30 nmole 1-1. The total As concentration exhibited a positive correlation with acidity, excess (non-sea-salt) sulfate, and nitrate, suggesting common atmospheric sources and fates from fossil fuel combustion. Contributions of As from marine sources appear to be minor. Reduced As (111) was found to comprise between 0 and 40 % of the total in selected events; the As(lI1)/(V)ratio may be indicative of the redox intensity (pE) and/or source of the associated storm system. Based on concurrent bulk (wet plus dry) collections, the total As deposition rate varied from 42.7 to 296 nmole m 2 mo- ~,with dry deposition estimated to comprise between 29 and 55 % of the total. Key word index: Acid rain, arsenic, tracers, chemistry of precipitation, pollution sources.
be sampled without significant ground-level contamination, which can often be a problem with groundWhile the utility of As as an elemental tracer of coal based aerosol and dry deposition measurements. combustion has previously been recognized (Walsh While atmospheric As can be derived from both et al., 1979b; Gordon, 1980; Rahn and Lowenthal, natural and anthropogenic sources (Pierson et al., 1984), studies to date have exclusively focused on the 1973; Brimblecombe, 1979; Lantzy and Mackenzie, ambient aerosol composition. However, when such an 1979; Walsh et al., 1979b), in the northeast U.S the elemental tracer approach is utilized to identify the atmospheric burden is dominated by industrial emissource(s) of acid rain, there are a number of distinct sions. The major source of such emissions is from the advantages to examining such tracers directly in pre- combustion of coal, including electric power genercipitation. First, in precipitation such tracers are ation and non-ferrous smelting (Walsh et al., 1979b). subjected to similar emission, transport and deposition As in coal fly ash exists primarily as the oxide, processes as the acidic components. Evidence suggests As(llI)20 3 (Crecelius, 1975; Turner, 1981), and like that fine atmospheric particulates, typical of trace sulfate, is associated with fine sub-/~m aerosols (Linton metal combustion condensates and acid aerosols, are et al., 1976; Walsh et al., 1979a). There is also evidence most effectively removed from the atmosphere by wet for the existence of an atmospheric vapor phase (Walsh deposition (Arimoto et al., 1983). Second, layering in et al., 1979a), which is perhaps related to biogenic the lower troposphere (leading to chemical stratifi- emissions of methylated forms (Johnson and Pilson, cation in the composition of the atmosphere) is particu- 1975). larly important in addressing the sources of acid Concentrations of As in precipitation reported precipitation, since long-range transport and in-cloud for various world-wide locations (as summarized nucleation processes occur in conjunction with upper by Galloway et al., 1982a) range from 0.07 to level air flow. While precipitation acts to vertically 77 nmole 1-1. Recent studies of As in rainwater from integrate the lower troposphere with respect to alti- the west coast of the U.S. and Hawaii (Andreae, 1980) tude, most aerosol measurements only reflect con- reveal relatively low concentrations (averaging ditions at or near ground level. Third, in contrast to 0.25 nmolel-~), consistent with open ocean aerosol aerosol measurements during periods of typical air measurements (Duce et al., 1975; Walsh et al., 1979a) flow, precipitation events at a given location tend to and onshore air flow. In contrast, precipitation and exhibit very specific storm trajectories, and in a aerosols collected downwind from a large copper meteorological sense, are very ephemeral features smelter exhibit relatively high (13-227 nmole 1-1) As (usually precipitation occurs less than 10 '.~oof the time). concentrations (Crecelius, 1975; Andreae, 1980). Thus, it is unlikely that aerosol measurements made Limited east coast measurements (Waslenchuk, 1978; during non-precipitating periods are representative of Galloway et al., 1982a) exhibit large variability the sources of acid rain precursors. Lastly, we have (0.4-27 nmole 1-1). Thus, due to variable and limited shown (Tramontano et al., 1987) that precipitation can representative data for As deposition over continental INTRODUCTION
937
938
JOSEPH R. SCUDLARK and THOMAS M. CHURCH
regions, attempts to obtain a global atmospheric mass balance for As (Lantzy and Mackenzie, 1979; Walsh et al., 1979b; Andreae, 1980) suffer from a large degree of uncertainty.
out of 90 total were analyzed, representing 90 ~'. of the total incident precipitation during this period. Due to volume limitations, only 15 of these event,~ werc analyzed for reduced As(Ill). Bulk deposition collections and measurements were conducted for the period October 1985-October 1986 only.
SAMPLING AND ANALYSES RESULTS AND DISCUSSION
Precipitation samples for As analysis were obtained on an event basis at the Lewes, DE, MAP3S/WATOX site using a wet-only sampler and rigorous trace metal sampling protocol (Tramontano et al., 1987). The sampling site is located on the mid-Atlantic coast, in a remote area of Cape Henlopen State Park (38 ° 46'N, 75°06'W). The Atlantic Ocean is located about 0.8 km to the east, while Delaware Bay is about 2 km to the northwest. The soil in the vicinity of the site is composed of well-sorted, coarse sand and the vegetation is predominantly scrub pine. Precipitation at this site is generally representative of the northeastern U.S. acid rain corridor, with an annual average pH of about 4.25 and average volume-weighted sulfate and nitrate concentrations of 24.5 and 18.9 pmole 1-~, respectively (Church et al., 1982; Dana and Easter, 1987). Due to its maritime location, many events inherit a variable marine component as well, with an average of 14 ',"oof the total sulfate being derived from sea-salt incorporation. Collections utilize a commercially available automatic collector (Aerochem Metrics, Inc., Bushnell, FL) modified with polycarbonate lid and Teflon-coated support arms, lining of the collection bucket with a heavy gauge (4rail) polyethylene bag liner, postacidification (0.4 ~."ov/v) of the sample in the collector with high-purity HCi to effect quantitative desorption, and the use of acid-cleaned liners and polyethylene storage bottles (Tramontano et al., 1987). Concurrent bulk deposition (wet plus dry) measurements were made using an acid-washed polyethylene funnel-bottle sampler which was continually exposed to the atmosphere. The funnel possesses extended vertical sides (32 cm) which are intended to limit aerodynamic effects on sampling efficiency. The funnel orifice was located about 2 m above ground level, approximately the same height as the wet-only collector. Arsenic speciation determinations were made using the d.c. plasma emission spectrometry procedure of Braman et al. (1977), except that high-purity HCI was substituted for pH adjustment in the total As analysis. Samples for major ions and reduced S(IV) were collected in parallel with the trace element samples and assayed according to standard MAP3S Network procedures (Dana and Easter, 1987). Due to oxidation of arsenite resulting from the prescribed acidification of trace metal samples, separate aliquots for reduced As(Ill) determination were obtained from the major ion sampler, and stored frozen until analysis. Field and laboratory blanks were established to verify the accuracy of our measurements. For the period June 1985-October 1986, 74 events with sufficient volume
In all samples analyzed, only inorganic As was detected. About 25 '!.,;of the samples were analyzed for organo-As species (methylarsonate and dimethylarsinate), selected to represent extremes in pH, season, and storm trajectory; in none of the samples were these species present above our detection limits for these compounds (0.10 nmolel ~ ~/. Despite the presence of relatively high concentrations of these biomethylated arsenic species in nearby coastal waters (Sanders, 1985) and their chemical stability at low pH values typical of local precipitation (Andreae, 1980), these compounds do not appear to make a significant contribution to the atmospheric deposition of As at our site. Concentrations of total inorganic As measured in precipitation events (Table 1) range from <0.07 to 14.95 nmolel - t , with an annual volume-weighted mean of 1.30 nmolel-~. Perhaps the most revealing observation is the correlation between total As with acidity (proton concentration) and with the acid anions, nitrate and excess (non-marine) sulfate (Figs la-c). These correlations suggest that As and the acid components share common transport and deposition pathways; similar observations have been made for Se at this same site (Cutter and Church, 1986). The consistent positive As intercepts in Figs la-c (0.61 __+0.16 nmole 1- ~) may reflect a regional background input of As from non-acidic sources, such as from crustal weathering or from biogenic emissions of volatile methyl arsenic compounds (Johnson and Braman, 1975). The concentration of each major ion (Junge, 1963) and trace element (Lindberg, 1982) in precipitation is influenced by many factors, including its atmospheric aerosol concentration, its in- and below-cloud scavenging efficiency, and the precipitation type, amount, duration, intensity, and antecedent dry period. With the acid components and As, a weak inverse relationship exists between the rainfall amount and concentration (an exponential decrease in concentration with increased precipitation), suggesting that their concentrations may be related to the quantity of precipitation. However, with the exception of Se (Cutter and Church, 1986) and NH~, no other major ion or trace element we have examined exhibits strong correlations with the acid species. Thus, in consideration of what is known about the atmospheric sources of As (Brimblecombe, 1979; Lantzy and Mackenzie, 1979; Walsh et al., 1979b), we believe these correlations reflect common emission sources, and are not simply the result of a common washout/dilution effect related to rainfall amount.
Deposition of arsenic and association with acid precipitation
939
Table I. Arsenic deposition at the mid-Atlantic coast of North America Volume-weighted average concentration in precipitation: Average wet depositional flux: Average (measured) dry depositionai flux: Average (calculated) dry depositional flux:
1.30 nmolel1.49 ,amolem- 2 a- l 0.61 ,amolem-2 a 1.15 ,amolem-2 a -j
Average (measured) total (wet + dry) deposition rate: Average total As in air: Washout ratio (,agkg- 1) precip./(,ag kg- ~) air: Crustal enrichment factors (AI): Precipitation Aerosols
2.10 ,amolem-2a 1 1.05 ng m - 3
110 25 31
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Fig. 1. Correlation of arsenic (nM) with (a)protons Lueql-~); (bl excess (non-sea-salt, sodium normalized) sulfate (,aM); (c) nitrate (/aM); and (d) sodium (,aM) in precipitation at the mid-Atlantic coast. (b) [A,], o.ozgz [sq,']. o.Tz r = 0.63,
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While some limited measurements of As in atmospheric aerosols indicate predominance in the oxidized pentavalent state (Waslenchuk, 1978), other studies (Crecelius, 1975; Andreae, 1980; Turner, 1981) have reported that As emissions in coal fly ash and from smelting activities are almost entirely in the reduced trivalent state, presumably As(III)20 3. In the precipitation samples we have analyzed for As(Ill), we have detected no more than 40'>o of.the total As present as arsenite (Fig. 2). In addition, a significant fraction of these samples exhibited As(Ill) concentrations below our detection limits ( < 0.007 nmole 1-1), though these results may be somewhat biased since only larger volume events were analyzed. No correlation was observed between the As(III)/(V) ratio and any other trace element analyzed [including the Se(IV)/(VI) ratio]; none of the samples analyzed for As(Ill) revealed detectable levels (<0.1 #M) of S(IV). In order to maintain the proportionality between the precipitation acidity and total As concentration (Fig. 1), the wide distribution in As oxidation states shown in Fig. 2 cannot be explained simply on the basis of temporal variability in the emission source strengths of the As redox species. Though no published data exist on the rate of arsenite oxidation in rainwater, rates reported for other aqueous systems (Johnson and
940
JOSEPH R. SCtlDLARK and THOMAS M. ('HURCH
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(1) In accord with the observed As-H * relationship (Fig. la), storms influenced by marine air masses (which are characterized by comparatively high pH values, typically 4.50-6.00), exhibit the lowest arsenic levels. (2) Direct measurements of storms exclusively of oceanic origin sampled at Lewes (Hurricanes Danny, Gloria, and Charley, tropical storm Henri) display relatively low As concentrations ( <0.40 nmolel ~). These results are corroborated by analyses of snow pit samples from Antarctica and Greenland at our laboratory and elsewhere (Weiss et al., 1975), which also exhibit low background As concentrations ( < 0.27 nmolel ~). However. as previously discussed, the low concentrations in oceanic storms may partially reflect increased dilution by typically larger volume events. (3) A lack of correlation between As and Na (Fig. ld) indicates that processes generating sea-sah aerosols do not contribute to As in precipitation. Based on seawater As concentrations reported for the western North Atlantic (Johnson and Pilson, 1972) and the average Na concentration in local precipitation (Church et at., 1982), the direct input of inorganic As from seasah incorporation (assuming no chemical fractionation) would be insignificant ( < 1 ",,).
Pilson, 1975; Cherry et al., 1979; Tallman and Shaikh, 1980; Oscarson et al., 1981), adjusted where possible to reflect a precipitation matrix (with respect to ionic strength, temperature, pH, and initial arsenite concentration), are not nearly sufficient to account for the observed As species distribution. Thus, it appears that the As speciation in precipitation is mediated by the redox poise established by atmospheric oxidative catalysts (e.g. 03, H202, u.v. light, other trace metals such as Mn and Fe) and reductants (e.g. SO2, HSO3) Based on the As-H ~ relationship described here (Oscarson et al., 1975; Andreae, 1980; Taliman and .(Fig. la), and the precipitation acidities reported for Shaikh, 1980). remote global locations [5.0 < pH < 5.6; Galloway et Based on the measured As(III), As(V) and H ~ al. (1982b)], we would predict open-ocean As concenconcentrations, it is possible to calculate an apparent trations in precipitation in the range 0.5~).6 nmole I - t. redox intensity, pE = - log{e- ]. The appropriate half- This estimate is in good agreement with measured cell reaction dictated by the p H - p E domain of precipi- (Andreae, 1980) and predicted (Walsh et al., 1979b) tation is precipitation concentrations for remote sites. A similar extrapolation based on average annual precipitation H3As(III)O3 + n 2 0 ~ H2As(V)O4- + 3H + + 2eacidities reported for the northeastern U.S. [4.0 < pH E = 0.649 V < 4.2; Dana and Easter 11987)] yields As concen[As(Ili)] trations < 1.8 nmolel- ~. On this basis, our predicted so that: pE = I 1.0 - 3,/2 pH - 1:/2 log [As(V)] concentration is about a factor of two less than the median average reported for rural U.S. locations [as (Cherry et al., 1979). reviewed by Galloway et aL (1982a)]. Though this The mean pE calculated accordingly (5.29 + 0.68, discrepancy might be explained in part by geographical n = 14) is about half that similarly estimated for the variability, some of the older data on trace metals in Se(IV)/(VI) couple [11.50 + 1.01, Cutter and Church, wet deposition are suspec! because of the lack of (1986)-], and approximates that reported for the proper precautions in avoiding sample contamination N O 2 / N O ~- couple in rainwater (Morgan, 1982). Thus, (Barrie et al., 1987). While there exists no widely accepted method for As speciation appears to equilibrate faster and is more "labile" in terms of its redox response during atmos- accurately measuring dry deposition by direct pheric transport, while Se speciation is more resistant methods, our bulk measurements provide an indirect estimate of the total (wet plus dry) As deposition. to oxidation (conservative). Overall, our results lead us to conclude that As in These data indicate that the total arsenic deposition precipitation has a dominant anthropogenic source. rate along the mid-Atlantic coast (Fig. 3) ranges from Arsenic of marine origin (such as that derived from 42.7 to 352 nmole m 2 mo ~, with an annual average natural volatile biogenic sources and/or fractionation of 2.1 #mole m 2 a 1. Dry deposition of As (estimated from seawater) appears to make a minor contribution as the difference between concurrent bulk and wetto the atmospheric depositional flux, which has been only collections) comprises between 2 and 77 "~, of the suggested by other researchers as well (Waish et al., monthly averaged total As deposition (Fig. 3). 1979b). There are several lines of experimental evidence However, because of the abnormally low amount of rainfall during the summer of 1986 (about 25 ".,, below which support .our contention:
Deposition of arsenic and association with acid precipitation
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J A S 0 ND J F MAMJ J 1985 I[Z2~ m. m ~11986 MONTH
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941
signatures for the midwestern U.S. developed by Dutkiewicz et al. (1987), but not Rahn and Lowenthars (1984) midwestern source region As:Se ratio. We are currently conducting a more thorough examination of the use of such key elemental ratios in aerosols and precipitation in conjunction with air parcel trajectory analysis to define regional chemical signatures relative to our Lewes, DE, sampling site. The degree of atmospheric enrichment of As in Lewes, DE, precipitation and aerosols can be compared with that of other metals based on calculated enrichment factors (EF), i.e.
E F (A1) = ( ~ l )
Fig. 3. Monthly trends in wet and dry arsenic deposition lnmole m- 2 mo- 1 at the mid-Atlantic coast.
average), our dry deposition rates for this period may be slightly over-estimated. Overall, on a seasonal basis the wet and dry As deposition rates parallel the patterns regularly observed with the acidic components (summer maximum, winter minimum). As summarized in Table 1, the average measured dry deposition rate (50.7 nmole m -2 t o o - i ) is only about half the rate calculated on the basis of the average measured As aerosol concentration at Lewes (1.05 ng m-3, Wolff et al., unpublished data; Zoller et al., unpublished data) and reported particle deposition velocity for As (0.24 cms-1; Pierson et al., 1973). This would suggest that (a) measurements from our bulk collector are not significantly compromised by contamination from resuspended soil or other fugitive debris; (b) despite the occurrence of As aerosols as sub-#m particles [ M M D = 0.63 _+ 0.24/~m; Walsh et al. (1979a)], coupled with acidified (pH -<_1.6) storage of samples prior to analysis, some particulate As may be refractory and not detected by our methods; and/or (c) due to aerodynamic effects, our bulk sampler did not efficiently collect dry fallout. Overall, it seems reasonable to assume that the dry deposition of As represents between 29 % (measured) and 55 % (calculated) of the total As deposition at our site, which is similar to values reported for most other atmospheric trace metals (Galloway et al., 1982). Based on principal component analyses of aerosol data, Wolff et al. (1986a)conclude that coal combustion emissions from the midwestern U.S. are the primary source of sulfate at our Lewes site. Since atmospheric Se is also recognized as a tracer of coal combustion emissions, it is noteworthy that the As:Se ratios in Lewes, DE, precipitation [0.59, this work; Cutter and Church, (1986)] and aerosols [0.55; Wolffet al. (1986a), Zoller et al. (unpublished data)] are nearly identical. This agreement indicates that these elements have similar washout ratios (i.e. precipitation scavenging efficiencies), so that precipitation appears to serve as an effective integrator of such trace element emissions. These ratios are consistent with the regional elemental
precip / ( ~ )
shal;
where the concentration of element "X" is compared in precipitation (or aerosols) to its relative crustal abundance (Turekian and Wedepohl, 1961) using A1 as a reference element. When such a comparison is made for trace elements based on aerosol and precipitation data from Lewes (Church et al., 1984; Wolff et al., 1986a), Bermuda (Jickells et al., 1984; Wolff et al., 1986b) and Enewetak Atoll in the South Pacific (Duce et al., 1983), the relative order and magnitude of EF values (Fig. 4) are nearly identical to those reported in a similar comparison between Bermuda and Antarctic aerosol data (Duce et al., 1975). Such spatial and temporal consistency for all elements suggest common mobilization and transport processes leading to global enrichment of the troposphere. Furthermore, the consistency between the aerosol and precipitation enrichment factors for most metals would suggest that precipitation is an effective integrator of the atmospheric particulate composition. Somewhat surprisingly, the EF values for As are consistently lower than most other metals, more closely resembling crustal-dominated elements (EF near unity) than its sister metalloid Se. While the EF values for Bermuda are in agreement with earlier results (Walsh et al., 1979a), those for Lewes are on the 5 ; ~s 4
I L~'s
• o~osoLs
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9~-2
Jos[ PH R. S('[IDLARKand THOMAS M. CH[ RCH
lower end of the range reported for other Northern Hemisphere urban, continental and coastal marine sites (Rahn, 1976: Walsh et al., 1979a). The EF values for As also do not conform 1o the general trend that elements which form oxides with the lowest boiling points (i.e. are the most readily volatalized) exhibit the largest degree of enrichment.
CONCLUSIONS 1. The average concentration o f As in precipitation at our mid-Atlantic site is 1.30 nmolel 2. O f the total (wet + dry) As deposition rate (2.1 l~mole m - 2 a - 1) from 29 ~,, (meas.) to 55 '."~i(calc.j is dry fallout. 3. Atmospheric As at this site has a d o m i n a n t anthropogenic source, and shares c o m m o n transport and deposition pathways with acidic c o m p o n e n t s in precipitation. 4. Marine-derived As appears to be a negligible ( < 10';~o) c o m p o n e n t in precipitation at our site. 5. Redox speciation of As in precipitation is highly variable and may be indicative o f the oxidation intensity of the attendant air mass. Acknowledgements This work was supported by the U.S. Geological Survey (Delaware State Water Resources ProgramJ, the U.S. Department of Energy (MAP3S Network) and U.S. NOAA Air Resources Laboratory (WATOX Program). We thank P. Salevan for sample collection and computer assistance, Dr David L. Johnson for valuable comments on this manuscript and the loan of instrumentation, and W. B. Lyons and the Glacier Research Group for providing polar snow samples. This is a contribution to the WATOX Program. REFERENCES
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